Such an arrangement, is employed in telephone systems, disclosed in Netherlands Patent Application No. 7903664.
The control stage comprises two output transistors which are connected to a wire of a subscriber's line. The control stage receives supply current from a supply source one terminal of which has a positive voltage with respect to a reference potential, commonly referred to as ground, and the other terminal of which has a negative voltage with respect to ground. This result is that in proper operating conditions the voltage at the relevant wire of the subscriber's line has a value which is located between the values of the positive and the negative supply voltage. Owing to external influences, such as, for example, inductances due to lightning or in case of a short circuit with mains the subscriber's line may be subjected to an overvoltage of, for example, 2000 V. This may cause damage to the control stage and other equipment coupled thereto. A known manner of protection is the provision of a protection circuit, designated primary protection circuit here, between each individual wire of the subscriber's line and a safety ground and between the subscriber's line wires themselves. This protection is not sufficient as the current flowing through the protection circuit to the safety ground still produces, in combination with the series impedance of the protection circuit, a residual overvoltage on the control stage. This residual overvoltage may cause damage to the control stage, particularly if said stage is implemented in integrated form.
The invention has for its object simply and efficiently to prevent damage by residual overvoltage. This manner of protection is particularly suitable for control stages of subscribers' circuits which are wholly or partly implemented in integrated form. At the same time this manner of protection is independent of the contruction of the integrated circuits, both as regards structure and the integration technology used.
According to the invention, the control stage of the type mentioned in the opening paragraph is characterized in that an overvoltage protection arrangement is provided comprising a first diode connected between the first supply terminal and the control stage, the supply current being applied to the control stage via said diode, and a second diode connected between the second supply terminal and the control stage, the supply current being discharged via said diode in such manner that normally both diodes conduct the supply current. Positioning the diodes thus results that in the event of a disturbance due to overvoltage no current can flow in the opposite direction through the base-collector junction of one of the complementary output transistors of the amplifier which is part of the control stage, in the event that the bases are interconnected. Blocking this current path results in the other output transistor not being capable of conducting current, as its base current is zero. Positioning the diodes thus has the advantage that the two output transistors are protected from damage owing to the flow of current in a direction opposite to the direction of conduction as well as from damage by an excessively high current in the direction of conduction. In the event that the bases are not interconnected, there is a possibility that a current path is created via one of the transistors owing to emitter-base breakdown and conduction in the opposite direction. This would result in that current can flow through the central supply source of the telephone system in a direction opposite to the direction in normal operating conditions. A further advantage, both for the case of interconnected bases and for the case wherein the bases are not interconnected, is therefore that the said diodes prevent reverse current from flowing through the central supply source of the telephone system. A resultant increase of the terminal voltage of the supply source which results in damage to the equipment connected thereto is also prevented in this way.
In the event of overvoltage, the provision of the said diodes prevents current from flowing through the output transistors with interconnected bases, which current flow may result in the formation across these transistors of a voltage which produces breakdown in the emitter-collector path whereby damage occurs. Using output transistors whose bases are not interconnected entails the risk of base-emitter breakdown in the event of overvoltage. Therefore, for both configurations, the control stage of the type defined in the opening paragraph is characterized in accordance with a further measure of the invention in that the safety device comprises a third diode which has one side coupled to that end of the first diode which is not connected to the first supply terminal and its other side connected to the output of the amplifier, and in that the safety device comprises a fourth diode which has one side coupled to that side of the second diode which is not coupled to the second supply terminal and its other side connected to the output of the amplifier, these third and fourth diodes normally being non-conducting. These two diodes limit the voltage difference across the output transistor to the diode forward voltage thus preventing reverse conduction and/or base-emitter breakdown and resultant damage from occurring. The control stage of the type defined in the opening paragraph comprises an external output resistor and in accordance with a further measure of the invention said control stage is characterized in that the safety device comprises a first voltage-limiting element which is connected between the side of the first diode which is not connected to the output of the amplifier and a point of constant potential and in that the safety device comprises a second voltage-limiting element connected between that side of the fourth diode which is not connected to the output of the amplifier and a different point of constant potential. This configuration has two advantages. On the one hand the electric charge associated with an overvoltage can be discharged by one of the voltage-limiting elements. On the other hand advantageous use can be made of the presence of the external output resistor of the amplifier. Namely, because of the fact that in the current discharge path formed by the external output resistor, the third or fourth diode and the first and second voltage-limiting element, respectively, substantially the entire residual overvoltage flows across the external output resistor, so that the output of the control stage is protected from overvoltage.
In accordance with a further measure of the invention the control stage of the type defined in the opening paragraph is characterized in that the third diode is connected to the first diode, that the fourth diode is connected to the second diode, that the safety device comprises a first voltage-limiting element which has one side connected to a point of constant potential and its other side coupled to the output of the amplifier via a fifth diode and that the safety device comprises a second voltage-limiting element which has one side connected to a different point of constant potential and its other side coupled to the output of the amplifier via a sixth diode, such that the fifth and sixth diodes are normally non-conducting. Directly connecting the third diode to the first diode and the fourth diode to the second diode is advantageous if the control stage is wholly or partly implemented in integrated form, the components being insulated from the substrate by means of what is commonly referred to as a junction insulation. The fourth diode then has an additional protective function, namely to prevent the diodes which are formed by the junctions between the substrate and the components located therein from becoming conductive. This conduction may occur at a negative over-voltage. The fourth diode must then be dimensioned such that it becomes conductive at a lower voltage than the diodes in the integrated circuit.
In the event of overvoltage the charge associated therewith can be discharged via the fifth diode and the first voltage-limiting element or via the sixth diode and the second voltage-limiting element. Also here the combination of the external output resistor, the fifth or sixth diode and the first and second voltage-limiting element, respectively provides an additional protection of the output of the control stage.
The control stage of the type defined in the opening paragraph is characterized in accordance with a further measure of the invention in that a seventh diode is provided, which is connected between the signal non-inverting input of the amplifier and the connecting point between the second voltage-limiting element and the sixth diode, the seventh and eighth diodes being arranged such that they are normally non-conducting. A feedback resistor is provided between the subscriber's line and the signal non-inverting input. Via this feedback resistor an overvoltage disturbance on the subscriber's line may cause damage to semi-conductors which are coupled to the signal non-inverting input of the amplifier. By means of the said feedback resistor the seventh and the eighth diodes limit the voltage on the signal non-inverting input to a range of values which has the positive and the negative supply voltage as its limits. Since a conducting diode has a very low resistance, substantially the entire overvoltage flows across the said feedback resistor. The advantage of the configuration used here is the useful additional use which is made of the feedback resistor which is already present.
It is possible that the control stages are damaged by an overvoltage difference between the wires of a subscriber's line to which the control stages are connected. This overvoltage difference may have such a value that the protection means of each individual control stage do not respond. In order to eliminate the disadvantages resulting therefrom an arrangement is provided which comprises, connected to each wire of subscriber's line a control stage as claimed in any of the claims 3, 4, 5 or 6 and which is characterized in that this arrangement comprises at least a third voltage-limiting element one side of which is connected to that side of the first voltage-limiting element which is not connected to a point of constant potential and the other side of which is coupled to that side of the second voltage-limiting element which is not connected to a point of constant potential. This manner of connecting the third voltage-limiting element has the advantage that said element functions as a protection from an overvoltage difference between the two wires of a subscriber's line, the first and the second voltage-limiting elements being jointly used by the two control stages.